A better understanding of mammalian gonad development and sex determination is important for both biomedical and basic scientific reasons. A number of sex determination genes have been identified and characterized; however the position and relationship of these genes within the pathway remain to be defined. Many of these genes are implicated in pathological processes in humans, and have essential roles in the normal development of organs other than the gonads. In humans, heterozygous mutations in WT1 (Wilms' tumor-1) or SF1 (Steroidogenic factor-1) are associated with XY sex reversal. In contrast, on most genetic backgrounds XY mice heterozygous for null alleles of Wt1 or Sf1 (Wt1+/- or Sf1+/-) are normal males. Preliminary analysis, however, indicates that on a C57BL/6J-Y AKR (B6-Y AKR) genetic background both Wt1+/- and Sf1+/- mice are XY sex reversed. These heterozygous mice offer distinct advantages over homozygous null mice in investigating the roles of Sf1 and Wt1 in sex determination and gonadogenesis because their gonadal phenotype is less severe and they are viable. We hypothesize that the B6 alleles of Wt1 and Sf1 are hypomorphs and that this reduced function causes decreased expression of Sry, the Y chromosome testis determining gene. We also hypothesize that Wt1 B6 and Sf1 B6 function as hypomorphs only when other interacting genes are homozygous for B6 alleles. These hypotheses will be tested with two specific aims: 1) Determine if Wt1B6 and Sf1 B6 function as hypomorphs and if so, determine why, and 2) Determine the molecular mechanism of sex reversal in B6 XY AKRWt1+/- and B6 XY AKRSf1+/- mice. To accomplish these aims, morphogenesis and marker gene expression in fetal gonads will be examined to determine at what point testis determination becomes aberrant and which genes are affected. Genetic crosses will be used to ascertain if Wt1B6 and Sf1B6 themselves are hypomorphs or if they act as hypomorphs only in cooperation with one or more B6-derived genes. If the latter is true, these modifier genes will be genetically mapped as a first step to identifying them. Furthermore, comparative DNA sequence analysis, and a targeted mutation approach in mice, will be used to identify the changes that cause Wt1B6 and Sf1B6 to have reduced function.